Hi John-
Thanks. You were right--AMOS was not reading the sample size correctly
(i.e., I was not telling it correctly). When I corrected the problem, I
got the same estimates. Thanks for pointing me in the right direction.
Anthony
John Fox wrote:
Dear Anthony,
sem() does FIML estimation, not 2SLS, and so it's hard to understand
why you're getting "nearly identical" parameter estimates but very
different coefficient standard errors and model chi-squares. In fact,
unless the problem is very ill-conditioned, the parameter estimates
should be the same within rounding error, as should the model
chi-square. There is some room for small differences in the standard
errors -- sem() uses a numerical Hessian and I'm not sure what AMOS
does -- but large differences are indicative of some problem.
I suspect that you're not fitting quite the same model in sem() and
AMOS.
I hope this helps,
John
On Mon, 02 Feb 2009 14:50:26 -0600
Anthony Dick <ad...@uchicago.edu> wrote:
Hello-
I am using R to build my initial models, but need to use AMOS to
compare the models of two groups (adults vs. kids). The problem is I
am getting different results with R and AMOS for the initial models
of the separate groups (and the R results make more sense).
The parameter estimates (path coefficients and variances) from both
programs are nearly identical, but the model chi-squares (and
significance estimates of the parameter estimates) are different. I
am using Maximum Likelihood in AMOS. R I think defaults to
two-stage-least squares estimate, and AMOS 16 does not implement
2SLS.
I am using fMRI data, so the error variances are likely correlated,
and the data non-normal to varying degrees. Is 2SLS the better way to
go for these kinds of data?
Is there a way to change the default method for R sem? I couldn't
find this in the ?help. I note I have run some of the AMOS examples
in R and have gotten identical results across platforms, so I believe
the problem is not in specifying things incorrectly across platforms.
Also, the dfs are identical for both analyses. I must use AMOS to do
model comparison (and thus maximum likelihood), but would like to
achieve similar results across platforms for the basic models before
I proceed, and would like to track down the reason for the
difference.
Thanks,
Anthony
--
Anthony Steven Dick, Ph.D.
Post-Doctoral Fellow
Human Neuroscience Laboratory
Department of Neurology
The University of Chicago
5841 S. Maryland Ave. MC-2030
Chicago, IL 60637
Phone: (773)-834-7770
Email: ad...@uchicago.edu
Web: http://home.uchicago.edu/~adick/
______________________________________________
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PLEASE do read the posting guide
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--------------------------------
John Fox, Professor
Department of Sociology
McMaster University
Hamilton, Ontario, Canada
http://socserv.mcmaster.ca/jfox/
--
Anthony Steven Dick, Ph.D.
Post-Doctoral Fellow
Human Neuroscience Laboratory
Department of Neurology
The University of Chicago
5841 S. Maryland Ave. MC-2030
Chicago, IL 60637
Phone: (773)-834-7770
Email: ad...@uchicago.edu
Web: http://home.uchicago.edu/~adick/
______________________________________________
R-help@r-project.org mailing list
https://stat.ethz.ch/mailman/listinfo/r-help
PLEASE do read the posting guide http://www.R-project.org/posting-guide.html
and provide commented, minimal, self-contained, reproducible code.
